1. Field of Invention
This invention relates to ultrasonic devices and methods for on-line monitoring of industrial materials during die casting, molding and extrusion processes at elevated temperatures.
The monitoring properties during processing are the surface disturbance of the molten metals and solidified particles in the shot sleeve of the die caster, flow front, shrinkage and temperature of the processed part in the cavities of the die casting and molding machines, and viscosity and temperature of the extruded polymers. These properties can then be used as the process control parameters for the process optimization. The ultrasonic devices for the monitoring are solid ultrasonic waveguides, also called buffer rods, which are inserted into the processing devices such as the shot sleeve of the die caster, die of the die caster, mold of the injection molding machine or barrel of the extruder, and piezoelectric ultrasonic transducers made of high Curie temperatures and directly deposited on the external surfaces of the shot sleeve, die, mold and barrel by a sol-gel technique. These ultrasonic devices are operated in the reflection geometry in which only one side access of the processing machines is necessary.
2. Description of the Prior Art
In the process of die casting, firstly metal is heated, melted and conveyed to the shot sleeve which is a container, through the runner which is a flow channel and the gate. It is then fed into the cavity of a die which has a unique shape for a designed production part. After filling, the part is cooled and solidified inside the die until it is ejected out at some stage and another run starts.
When the molten metal is in the shot sleeve, little surface disturbance and solidification should exist before the injection. Such a surface disturbance and solidification may significantly degrade the quality of the cast product.
Then the molten metal is injected by the hydraulic cylinder (plunger or piston) into the die cavities through the runner and the gate, the flow of the molten metal advances with certain paths inside the die. Usually, a high pressure is applied through the plunger and the molten metal fills the cavity at a high speed with a strong force. When the cavity is filled, the filling pressure should be immediately switched to a much higher intensification pressure. With the monitoring of the advancement of the flow front a smooth transition from the filling stage to intensification stage can be achieved by switching the filling pressure to a relatively high intensification pressure.
After the filling stage, the part is subjected to an intensification pressure through the gate as it is cooled and solidified. For the die casting process, the gate is frozen soon after the filling, and intensification could be applied right after gate solidification. The ability to detect whether the runner or gate is frozen or solidified or not will help to establish the effectiveness of intensification and thus feeding in the cavity.
When the solidified material begins to shrink through its thickness, a gap is likely to be formed between the die wall and the cast part. Due to the formation of the gap, there might be a significant thermal contact resistance between the part and the cavity wall, and it can reduce the heat transfer efficiency which affects the production cycle. Detection of the gap formation may provide more information about the heat transfer inside the parts and improve the physical understanding of thermal contact resistance of the gap. Furthermore, the detection of the gap can also reveal the uniformity of the cooling throughout the part which is also an important issue affecting the part quality.
In order to increase the production speed the cast part in the die needs to be cooled efficiently and properly in order that it can be ejected in the shortest allowable time frame. Thus, it is desirable to know the temperature and the temperature profile of the cast part in the die cavity and cooling mechanism of the die. This information can help cooling line design, ejection time prediction, etc. In order to probe the temperature profile of the cast part in the cavity of the die three information; namely the surface temperature, the heat flux and the average temperature of the part, are required. One single sensor which can obtain these three information is highly desired.
Therefore, there are four essential monitoring tasks desired to improved the die casting process. They are (a) surface disturbance and solidification monitoring when the molten metal is in the shot sleeve before injection, (b) flow front monitoring during the filling stage, (c) monitoring of the gap development which is caused by the shrinkage of the part during the cooling and (d) monitoring of the temperature of the part that significantly affects the properties such as the warpage, shrinkage and density distribution of the cast part. Performing all these monitoring can lead to an improved on-line quality control system and improve the basic understanding of thc die casting process.
In pressure die casting, German Patent, "Sensoreinhelt", DE 44-40-070 A1, Jun. 22, 1995 proposes a device for detecting metal flow based on an electrical contact between two electrodes separated by an insulating ceramic ring making up the metal flow sensor. Once the metal flow passes these two electrodes, the electrical resistance sharply decreases, and hence flow can be detected. This sensor can not be used to measure the flow front of an insulator such as polymer. An improved sensor which overcomes the above problem and carries out other monitoring tasks such as gap development, temperature and viscosity of the cast part is highly desired.
Using pressure sensors which monitor the cavity pressure to detect the gap can be one approach. The onset of gap corresponds to the instant when the pressure drops to zero. However, a precise detection can hardly be achieved because the pressure sensors are made to measure the peak pressures and significant errors may occur, particularly in the low pressure range which corresponds to the critical period when the gap starts to be formed.
In squeeze casting which is a type of die casting the turbulence of the molten metal in the shot sleeve and gas entrapment in the cast part can be significantly reduced. The above four desired monitoring tasks are still desired for the improvement of the casting.
Semi-solid metal casting is another modern type of die casting. It involves injection of, for instance, aluminum alloy in the form of semi-solid slugs (60% solid, 40% liquid) having a consistency of toothpaste into a die cavity. Since semi-solid metal casting operates at about 100.degree. C. lower temperature than the normal casting in which the metal is completely molten, it uses less energy and suffers only a fraction of the solidification shrinkage. For this casting ultrasonic monitoring tasks mentioned above are also desired for the improvement of the process.
Bolton et al. (U.S. Pat. No. 5,161,594, Nov. 10, 1992) reported a tie bar monitoring system using ultrasound for a die casting machine. This previous art is comprised of an ultrasonic device that monitors changes in tie bar length. Such an ultrasonic system will sound an alarm and/or shut down the die cast machine if bending in individual tie bar exceeds a predetermined limit. Such a system cannot perform the desired tasks mentioned above for die casting of metals.
It is understood that polymer injection molding is very similar to the die casting of metals except the material processing temperature is lower. For instance, in the die casting of aluminum and magnesium alloys, the melt temperature of these alloys is in the range of 650-750.degree. C. and that of the polymer in the injection molding is below 350.degree. C. However, there are some differences. For instance, in polymer injection molding machines there is no similar container like shot sleeve in the die casting machines but there is a polymer extrusion process before the injection of polymers. Therefore there are still four essential monitoring tasks for the polymer injection molding.
For the polymer extrusion process L. Piche et al (U.S. Pat. No. 5,433,112, Jul. 18, 1995) discloses the application of ultrasound to characterize a polymer flow. In this previous art the measurement geometry is in the transmission mode for which two side accesses of the extruder are required and only longitudinal waves are used for the measurements. Because the screw of the extruder blocks the transmitted ultrasonic energy, in the monitoring can be only carried out at the exit of the extruder.
Furthermore, after the filling the pressure should be immediately switched to a much lower packing pressure in order to avoid the part splashing in the polymer injection molding. In die casting, the pressure in the cavity is normally increased during the solidification, that is called intensification, to allow more material to enter the cavity in order to compensate for volumetric shrinkage. However, in each case, after the gap is developed, and especially after the gate is frozen, the holding pressure will not further affect the part. Thus keeping the holding pressure through the gate is not necessary. A dynamic control of the hold time of injection molding and die casting may be achieved by monitoring the gap development at the gate location resulting in parts with a higher quality (denser, less porosity, etc.) and an optimal production cycle time. In addition, temperature profile of the part is also an important information for the process control of polymer injection molding.
The above information is also desired for gas assisted injection molding and co-injection molding. In the gas assisted injection molding a gas is injected into the core of the molded part which consists of a hollow core and an outer layer in order to reduce the material used and the weight, and preserve the desired shape of the part because of the reduced shrinkage. In co-injection molding a low cost material is injected as the core material. For these two injection molding processes the flow front of the gas and core material are also desired to be monitored.
A. J. Bur et al (U.S. Pat. No. 5,384,079, Jan. 24, 1995) discloses used an optical method to detect the thermodynamic phase transitions during polymer injection molding. This optical method not only needs to add a fluorescent dye which is a foreign material into the mold but also use glass optical fibers as the detectors of which the thermal characteristics are different from that of the mold material which is generally steel. A sensing mechanism which does not add foreign material, has same thermal characteristics as the mold and performs the desired three monitoring tasks is highly desired.
The ability of the ultrasound to interrogate noninvasively, nondestructively and rapidly the surface and internal regions of material objects is clearly desirable for a modern injection molding process control. Such a control should not disturb normal processing conditions and consistent product properties in a batch or continuous process and, at the same time, it should acquire the desired information fast enough to provide efficient feedback to the process control. Recent advances in transducer materials, microprocessors, digital signal processing and measurement techniques allow the data to be obtained and analyzed rapidly, reliably and economically, and make ultrasound a practical tool for on-line production monitoring.
In the publication "In situ monitoring of molding processes using laser-based ultrasound", Review of Progress in Quantitative Nondestructive Evaluation, vol. 13, Plenum Press, New York, pp.2237-44, 1994, Addison, Jr. et al recognized the capability of ultrasound and reported a laser ultrasonic method to monitor the flow front of the molten polymer in a compression molding and a resin transfer molding machines. This laser ultrasonic method is non contact. However, the repetition rate of the high power laser pulses which generate ultrasound in the material is around 100 Hz which is too slow for the monitoring of the flow front and the gap development in the metal die casting and polymer injection molding machines.